This invention relates to the provision of corrosion resistant, hydrophilic coatings for surfaces of aluminum articles. In particular aspects it is directed to coating compositions, methods of applying them, and aluminum articles having surfaces so coated. Illustrative examples of articles that may be beneficially coated in accordance with the invention include, without limitation, aluminum foil, and aluminum sheet from which various types of components and products are formed. The term "aluminum" is used herein to refer to aluminum metal and aluminum-based alloys.
For certain purposes, aluminum articles, e.g. sheet articles, are desirably provided with hydrophilic surfaces. One commercially important example is the aluminum fin stock (sheet aluminum, in final gauge) from which fins are made for heat exchangers in air conditioners. Water condensing on the surfaces of the closely spaced fins in an air conditioner tends to accumulate in the form of drops that impede airflow between the fins, thereby reducing heat exchange efficiency. This problem can be overcome by producing the fins from fin stock having a hydrophilic coating on its surfaces; the coating allows water to drain from the fin surfaces and largely prevents the development and retention of airflow-obstructing drops. Since the environment of use of the fins is relatively severe, it is desirable that the coating also afford protection against corrosion.
A satisfactory hydrophilic and corrosion-resistant coating for fin stock or the like must be smooth and nonporous with relatively uniform thickness. To these ends, as well as to ensure that it remains durably on the fins which are formed from the stock, a strong bond must be formed between the material of the coating and the coated aluminum surface; otherwise, as the coating is dried or cured with heat after application, it may tend to move relative to the surface, developing regions of differing thickness and/or shrinkage cracks. In addition, the coating must maintain good corrosion resistant and hydrophilic properties over extended periods of exposure to water; it should be nontoxic and environmentally acceptable in application, use and recycling, as well as being inexpensive, easy to apply, and free from tackiness or stickiness.
Heretofore, a variety of hydrophilic coating systems have been proposed for imparting hydrophilicity to aluminum surfaces. A serious difficulty presented by many of the known coating formulations is that oxide material (such as silica or alumina or their precursors), included therein to impart hydrophilicity, renders the produced coatings abrasive. The abrasive character of the coatings causes increased wear of the tooling used in air conditioner fabrication, i.e., incident to forming or other operations performed on fin stock thus coated.
It is also known that polymers of a polar nature, such as polyvinyl alcohol and polyacrylic acid, can provide satisfactorily hydrophilic films. Such films, however, tend to absorb water and swell, and then afford little or no corrosion resistance. Attempts have been made to stabilize the polymers by cross-linking but these attempts have not yet achieved successful results.